Vaccinia virus is the prototype for the orthopoxvirus of the poxvirus family. Its biology and replication have been described extensively by B. Moss ("Poxviruses", in Comprehensive Virology, eds. H. Fraenkel-Conrat and R. Wagner, Plenum Press, New York, Vol. 4, pp. 405-474 [1974]; "Poxviruses", in Molecular Biology of Animal Viruses, ed. D. P. Nayak, Marcel Dekker, New York, Vol. 2, pp. 849-890 [1978]; "5' End Labelling of RNA with Capping and Methylating Enzymes", in Gene Amplification and Analysis, eds. J. G. Chirikjian and T. S. Papas, Elsevier, North-Holland, Vol. 2, pp. 254-266 [1981]; "Principles of Virus Replication: Poxvirus", in Human Viral Diseases, eds. B. N. Fields, R. Chanock, R. Shope and B. Roizman, Raven Press, New York, in press [1984]). While several types of animal DNA viruses with large enomes have been used as cloning vectors, including adenovirus, herpes simplex virus and vaccinia virus, only recombinants of the latter have expressed foreign genes while retaining complete infectivity. Meanwhile great experience has been gained with the use of vaccinia virus as a live vaccine. Its wide host range, large capacity for foreign DNA and inability to induce oncogenic transformation are all features enhancing the potential of vaccinia virus recombinants as live vaccines. AN updated review of the use of recombinant vaccinia viruses as live vaccines has been given by G. L. Smith et al. (Biotechnology and Genetic Engineering Reviews 2, 383-407, [1984]), including a description of the biology of recombinant vaccinia viruses and the expression of foreign genes under the control of vaccinia promoters.